Field of the Invention
The invention relates to a polishing device and a polishing cloth for grinding or polishing semiconductor substrates and in particular for machining semiconductor substrates, in which end point control of the grinding or polishing operation is necessary.
Typical devices for grinding or polishing wafer surfaces, trench fillings, metal plugs, interoxide layers or the like of semiconductor substrates generally include a rotatable polishing table with an elastic overlay which is also called a polishing cloth or pad. A polishing agent is typically applied to the polishing cloth. In chemical-mechanical polishing (CMP), the polishing agent includes not only grinding or polishing particles but also chemical additives, which are selected on the basis of which layer is to be removed. The chemical agents reinforce the removal process.
The semiconductor substrate to be machined, for instance a silicon wafer, is generally guided on the polishing table by a rotating carrier that rotates in the opposite direction from the polishing table.
To enable selective control of the grinding or polishing operation, the polishing device generally includes a measuring device with which the end point of the machining can be ascertained. Examples of such measuring devices are optical devices for determining the layer thickness or the removal rate, or devices for temperature determination by means of infrared detectors. Often, the measuring device is integrated into the polishing table. In that case, a window is provided above the measuring device in the polishing cloth, the window being transparent to the wavelength range employed in the measurement. In order not to impair the polishing operation, it is thereby necessary to adapt the window material to the material which forms the polishing cloth. The way in which the window is integrated into the polishing cloth must also be such that the quality and service life of the windowed polishing cloth is substantially equivalent to that of a normal polishing cloth for a CMP process. It should also withstand the same mechanical and chemical stresses.
Windowed polishing cloths of this kind as a rule comprise an upper polishing cloth into which a window of transparent plastic is integrated, and a polishing cloth lying beneath it. The window material has mechanical properties comparable to those of the upper polishing cloth. The transparent window is typically larger than the associated opening in the polishing cloth below and is glued to the opening edge of the lower polishing cloth.
By way of example, such an configuration is available as a windowed polishing cloth IC1000 on a SUBA IV polishing cloth, both made by the Rodel company (stacked pad).
The prior art windowed polishing cloths are hard polishing cloths, which while they have good planarizing properties nevertheless readily cause scratches in the substrate. Such hard polishing cloths are therefore used primarily in an initial polishing stage for removing relatively large layer thicknesses. After that, the thus-treated substrate is repolished with a soft polishing cloth in a second polishing stage, in order to remove scratches. However, if only slight layer thicknesses are to be removed, then hard polishing cloths can generally not be used. In that case, soft polishing cloths must be used from the very beginning. Inserting windows into such soft polishing cloths is problematic, however, and reliable soft polishing cloths with an integrated window of plastic material are not currently available. This means that prior to this invention even a combination of a polishing table with an integrated optical measuring device and a soft polishing cloth with an integrated window could not be used.